1. Field of the Invention
The present invention relates to a novel method of isomerization of dimethylnaphthalenes. More particularly, the present invention relates to a novel method of isomerization of dimethylnaphthalenes which are utilized for the preparation of 2,6-dimethylnaphthalene which is, in turn, useful as the material for the preparation of 2,6-naphthalenedicarboxylic acid.
2,6-Naphthalenedicarboxylic acid has important industrial application as a raw material for high performance polyester, such as polyethylene naphthalate. Fibers and films made of polyethylene naphthalate have excellent tensile strength and heat resistance.
2. Description of the Prior Art
2,6-Dimethylnaphthalene is the material utilized for the preparation of 2,6-naphthalenedicarboxylic acid and a high degree of purity is required. (Dimethylnaphthalene will be abbreviated as DMN in the following descriptions.)
In DMN, there are 10 isomers having the two methyl groups at different positions in the molecule. It is required that 2,6-DMN comprising substantially no other isomers be prepared in large quantities with a reasonably economical efficiency.
In the isomerization of DMN, it has been known that neither the isomerization between the adjacent .beta.-positions nor the isomerization by the transfer of a methyl group on one ring to the other takes place more easily than the isomerization between an .alpha.-position and its adjacent .beta.-position. Thus, with respect to the isomerization, DMN can be classified into the following four groups. The isomerization between the following different groups takes place less readily than that within the same group.
A group: 1,5-DMN, 1,6-DMN, 2,6-DMN PA0 B group: 1,8-DMN, 1,7-DMN, 2,7-DMN PA0 C group: 1,4-DMN, 1,3-DMN, 2,3-DMN PA0 D group: 1,2-DMN
2,6-DMN can be prepared by a method of isomerization and isolation after the methylation of methylnaphthalene or naphthalene or by a method of the isolation of 2,6-DMN from tar fractions or petroleum fractions. The fractions or the reaction products contain almost all of the isomers of the four groups and isomerization between the isomers of the different groups is necessary for the efficient preparation of 2,6-DMN from such kind of fraction or reaction products. As a method of isomerization between the isomers of the different groups, for example, a method utilizing a specific zeolite is described in Japanese Laid-open Patent Publication Showa 59-88433. However, the method requires high temperatures for the isomerization and side reactions other than the desired isomerization, such as disproportionation, takes place to a large degree, leading to low yields of the desired 2,6-isomer.
When a mixture of various DMN isomers is isomerized to prepare the 2,6-isomer, the mixture inevitably contains many isomers which are difficult to isomerize to the 2,6-isomer. The yield of 2,6-isomer is low and the isolation of the desired isomer from many other isomers is necessary. Thus, the preparation of the 2,6-isomer in such cases is not efficient.
Another example of the conventional methods for the preparation of DMN is as follows. A method for preparing o-tolylpentene-2 by the reaction of o-xylene and butadiene in a high yield is described in Japanese Laid-open Patent Publication Showa 49-134634. A method for preparing 1,5-dimethyltetraline by the cyclization of o-tolylpentene-2 is described in Japanese Laid-open Patent Publication Showa 50-89353. A method for preparing 1,5-DMN in a high yield and in a high selectivity by the dehydrogenation of 1,5-dimethyltetraline is described in Japanese Laid-open Patent Publication Showa 48-76852. The combination of these methods gives 1,5-DMN from o-xylene and butadiene.
1.5-DMN belongs to the same group as 2,6-DMN and 1,5-DMN prepared above has the advantage that 2,6-DMN can be prepared from it without having to go through the difficult isomerization between different groups. Various methods have been proposed for the isomerization of 1,5-DMN to prepare 2,6-DMN. As an example of these methods, isomerization in gaseous phase utilizing silica-alumina as a catalyst is described in Japanese Patent Publication Showa 47-50622 and, as another example of them, isomerization in liquid phase utilizing a specific zeolite as a catalyst is described in Japanese Patent Publication Showa 58-004008.
However, in the liquid product obtained by the method described in Japanese Patent Publication Showa 47-50622, considerable amounts of the 2,7-isomer and the 1,7-isomer which belong to a group different from the group of the 2,6-isomer are contained and monomethylnaphthalene and trimethyl-naphthalene which are formed by the disproportionation reaction are also contained in considerable amounts even though the content of the 2,6-isomer is high.
The method described in Japanese Patent Publication Showa 58-004008 produces the 2,6-isomer in low yields even though side reactions, such as the formation of the 2,7-isomer and disproportionation, proceed to a lesser degree.
It is the actual situation at the present time that the 2,6-isomer can not be prepared, by utilizing conventional methods, from the 1,5-isomer in high yields while keeping the side reactions, such as the formation of isomers belonging to groups different from the 2,6-isomer group and disproportionation, to a low level.
When the 2,7-isomer is formed in the process of isomerization to the 2,6-isomer, the yield of the 2,6-isomer is naturally decreased and, furthermore, two component eutectic mixtures of the 2,6-isomer and the 2,7-isomer and three component eutectic mixtures of the 2,6-isomer, the 2,7-isomer and the 1,5-isomer are formed, resulting in the loss of the yield of the 2,6-isomer in the process of the isolation by crystallization after isomerization and in the decrease of purity of the product. Side reactions such as disproportionation reaction also cause a loss of the yield of the 2,6-isomer.